The basic function of a lagoon system is to adequately store and treat liquid waste, such as wastewater, livestock manure, etc., to control unwonted odors and for ultimate return to the environment. Anaerobic (no oxygen present) lagoons or basins store and treat high-strength wastewater and livestock manure. Anaerobic bacteria (requiring no oxygen) found in these basins can generate various odorous gases that ultimately discharge into the atmosphere. The emitted odorous gases can include hydrogen sulfide, ammonia, volatile acids, phenols, etc. The emitted noxious gases additionally may provide health concerns for surrounding human and animal populations. Aerobic bacteria (oxygen-requiring) can biologically destroy these odorous gases. In the presence of oxygen, aerobic bacteria can convert hydrogen sulfide into sulfate anions, ammonia into nitrite and nitrate anions, and volatile odorous organic compounds into carbon dioxide. The technical problem in such conversions involves providing a sufficiently large aerobic zone within an initially anaerobic basin to biologically destroy odorous gases without incurring high operating costs associated with aerating the entire anaerobic basin contents.
One method of creating an aerobic zone within an anaerobic basin is to convert the anaerobic basin into a facultative lagoon by aerating only an aqueous layer at the basin surface. Municipal and industrial wastewater treatment industries have used facultative lagoons for decades to meet treatment objectives with reduced aeration costs. Facultative lagoons typically consist of a basin in which solids in the wastewater and/or aqueous livestock manure settle to the bottom as a sediment layer that decomposes anaerobically. The term facultative describes the aerobic-anaerobic nature of the lagoonxe2x80x94an anaerobic bottom zone covered by an aerobic top zone. The biodegradable organic materials that do not settle can degrade aerobically due to oxygen present in the aerobic top zone. The aerobic zone depth and content constantly fluctuate with added waste products and changing meteorological conditions.
Field studies suggest that a xe2x80x9ccleanxe2x80x9d water layer an inch or so thick at the lagoon surface can drastically reduce odorous gas emissions. The top water layer can be kept xe2x80x9ccleanxe2x80x9d by outside-the-basin treatment (an aerobic wastewater treatment plant along side the anaerobic storage basin) or inside-the-basin treatment (facultative lagoon). Self-purification (i.e., preferential biodegradation of odorous compounds) in the aerobic zone requires dissolution of oxygen therein. A variety of sources can supply oxygen, including biological sources (algae growth), chemical sources (addition of hydrogen peroxide), and mechanical means (aeration). Dissolution of ozone also may facilitate biodegradation of certain odorous gases. Livestock and industrial anaerobic storage basins often have such high initial oxygen demands that biological and chemical oxygen addition is technically or economically prohibitive.
Mechanical devices can introduce oxygen into the aerobic zone from the atmosphere or oxygen gas as bubbles. Dissolution efficiency depends on the size and residence time of the added bubbles. Smaller bubble sizes (with more surface area and slower bubble rise velocities) and deeper submergence depths (depth of the bubble source below the lagoon surface) result in improved oxygen dissolution efficiencies. Thus, there are two competing mechanisms: overall aeration costs dictate minimum treatment volumes (i.e., smaller or shallower aerobic zones), while oxygen dissolution efficiency dictates greater water depths.
The aerobic bacteria found in the upper layer of a facultative lagoon oxidize the odorous compounds generated in the lower anaerobic layer. The wastewater-treatment approach to a facultative lagoon is to maintain aerobic conditions in the upper one to two feet of the water column. Oxygen concentrations of 0.5 to 2.0 mg/L ensure that oxygen is not the rate-limiting substrate in the biological oxidation of BOD (biological or biochemical oxygen demand) and ammonia. BOD is a primary indicator of the amount of pollution in wastewater. However, if the treatment objective is to reduce the emission of hydrogen sulfide, ammonia, and other odorous effluent gases from an anaerobic basin, then the traditional wastewater treatment approach to a facultative lagoon can be excessive in terms of operating costs.
Anaerobic storage basins and anaerobic treatment lagoons typically range in depth from about 8-ft to about 12-ft deep, generally in areas where land is readily available or relatively inexpensive. For various industrial and municipal applications, where land is more expensive or not readily available, anaerobic storage and treatment basins can be deeper (e.g., about 15-ft to about 18-ft deep). Anaerobic storage basins generate odors that are released by two general mechanisms: (1) diffusive transport across the water surface (volatilization), and (2) transport of odorous gases to the water surface within gas bubbles and subsequent bursting of gas bubbles at the water surface to release odorous gases to the atmosphere. Oxygen demand and aeration efficiency will determine the depth of the aerobic layer in a facultative lagoon.
Ideally, the operation of a facultative lagoon for odor control should supply just enough aeration to biodegrade the odorous compounds that would otherwise be released into the atmosphere. The low-cost operational goal of a facultative lagoon for emission control is to minimize the volume of water that needs aeration and to supply just enough aeration to biodegrade the target odorous compounds. It is difficult to substantially reduce the volume of water requiring aeration with a traditional facultative lagoon, because the aerators used to create the aerobic layer will generate a certain degree of mixing between the aerobic and anaerobic layers. This unavoidable mixing removes aerobic bacteria from the aerobic layer and introduces high strength wastewater into the aerobic layer. Hence, sufficient aeration capacity must be supplied to a facultative lagoon (1) to generate additional aerobic bacteria to replace those lost to the anaerobic layer and (2) to oxidize odorous compounds added to the aerobic layer by mixing.
A compartmentalized facultative lagoon comprises a lower anaerobic zone containing aqueous-based liquid organic anaerobic waste material with odor-causing compounds; an upper aerobic zone containing aqueous-based liquid material; a source of aeration positioned within the aerobic zone adapted and designed to aerate only the aerobic zone; and a porous barrier that compartmentalizes the zones from each other to eliminate macroscopic mixing between the zones while permitting passage of the odor-causing compounds from the lower to the upper zones. The compartmentalized facultative lagoon facilitates the aerobic biological conversion of the odor-causing compounds created within the anaerobic zone into olfactorily inoffensive compounds.
The porous barrier can be a porous membrane positioned horizontally between the zones. The lagoon can be about eight-twelve feet in depth and the porous barrier can be positioned about one-two feet below the surface of the liquid. The porous membrane can be less dense or more dense than water. The lagoon can include a biofilm on one or both surfaces of the porous membrane. A biofilm on an under surface of the porous membrane can be an anaerobic biofilm. A biofilm on an upper surface of the porous membrane can comprise aerobic bacteria located near an upper surface of the biofilm and anaerobic bacteria located near the porous membrane. The source of aeration can be an aeration device, such as an airlift aerator, a pump-driven aspirator that supplies air bubbles to the aerobic zone, a static air tube aerator, or a propeller-based aspirator.
A method of creating and maintaining a compartmentalized facultative lagoon comprises providing an anaerobic basin containing aqueous-based liquid organic anaerobic waste material; positioning a barrier below the surface of the basin material to compartmentalize the basin into an anaerobic zone below the barrier and an aerobic zone above the barrier, the barrier being porous to the odor-causing compounds generated by the material within the anaerobic basin while eliminating macroscopic liquid mixing between the zones; and positioning a source of aeration within the aerobic zone and aerating only the aerobic zone. The method facilitates biological aerobic conversion of the odor-causing compounds within the anaerobic zone into olfactorily inoffensive compounds.
Positioning the barrier can comprise positioning the porous membrane horizontally between the zones. Providing an anaerobic basin can comprise providing a basin about eight-twelve feet in depth. Positioning the barrier can comprise positioning the barrier about one-two feet below the lagoon surface. The barrier can be less dense or more dense than water. Positioning the barrier can comprise developing a biofilm on one or both surfaces of the porous membrane. A biofilm on an under surface of the porous membrane can be an anaerobic biofilm. A biofilm on an upper surface of the porous membrane can comprise aerobic bacteria located near an upper surface of the biofilm and anaerobic bacteria located near the porous membrane. The source of aeration can be an aeration device, such as an airlift aerator, a pump-driven aspirator that supplies air bubbles to the aerobic zone, a static air tube aerator, or a propeller-based aspirator